1. Field of the Invention
The present invention is generally related to a method for characterizing a membrane by a positron annihilation spectrometer and the sample holder thereof, and more particularly to a method for characterizing a membrane in a wet condition by a positron annihilation spectrometer and the sample holder thereof.
2. Description of the Prior Art
Pervaporation is a membrane separation process that combines the technologies of permeation and evaporation and has attracted a great deal of research to deal with its application in dehydrating aqueous alcohol solutions. High performance pervaporation membranes are specifically based on two criteria: the increase of permeation flux and the enhancement of selectivity. In this respect, asymmetric thin-film composite (TFC) membranes consisting of a thin dense top layer and a porous supporting layer are a feasible alternative. Since a thin selective skin layer has lower mass transfer resistance, the permeation flux can be enhanced during the pervaporation process. Nevertheless, the swelling problem inherently persists no matter which method is applied to modify membrane materials. Since pervaporation membranes are always in direct contact with the feed solution whose components are to be separated, the membranes inevitably experience a certain degree of swelling. There is a strong interaction between the feed solution components and the pervaporation membrane. To better understand such an interaction, it is essential to characterize the swelling behavior of a membrane.
However, the measurement of the extent of the swelling behavior in thin-film composite membranes encounters technical problems because most conventional instruments available are basically designed for application to free-standing membranes. With the advent of positron annihilation spectroscopy (PAS), the method for determining the swelling behavior of TFC membranes has been dramatically improved. However, characterizations of composite membranes are still done in their dry state. Therefore, a method for characterizing a membrane in a wet condition by a positron annihilation spectrometer is very important for the industries.